How
Old
Is The Earth?
Who
was
Martin Ryle?
Ryle,
on the right, soldering part of the antenna with his colleague Hewish.
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T
o
many philosophical minds,
the steady-state universe proposed
by Hoyle, Bondi and Gold had a major advantage over the big-bang expanding
universe. In their universe the overall density was kept always the same
by the continuous creation of matter. In the big-bang universe with its
radically changing density, various physical laws might not apply the same
way at all times. It would be impossible to extrapolate with confidence
from the present back to the super-dense origin of the universe.
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Tommy Gold (left) with Hermann Bondi (center) and Fred Hoyle (right), circa
1960.
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Steady-state
theory also had an observational advantage over big-bang theory in 1948.
The rate of expansion then observed, when calculated backward to an initial
big bang, gave an age for the universe of only a few billion years
well
below the known age of the solar system! That was certainly an embarrassment
for the big bang theory.
For
some time cosmologists had measured ideas against a "cosmological
principle," which asserted that the large-scale properties of the
universe are independent of the location of the observer. In other words,
any theory that put we humans at some special place (like the center of
the universe) could be rejected out of hand. Bondi and Gold insisted that
the universe is not only homogenous in space but also in time
it
looks the same at any place and at any time. They grandly called this
the "perfect cosmological principle," and insisted that theory
should be deduced from the axiom that we are not at any special place
in either space or time.
H
oyle
was less insistent
that the perfect cosmological principle
was a fundamental axiom. He preferred to have theory follow from a modification
he proposed to Einstein's relativistic universe, adding the creation of
matter. The two different steady-state theories had enough in common,
however, to be considered one for most purposes.
Much
of the later development of steady-state theory came in response to criticism.
In Great Britain, especially, scientists gave considerable attention to
elaborating the theory. Their arguments were largely of a philosophical
nature, with little appeal to observation.
T
he
cosmological debate
acquired religious and political
aspects. Pope Pious XII announced in 1952 that big-bang cosmology affirmed
the notion of a transcendental creator and was in harmony with Christian
dogma. Steady-state theory, denying any beginning or end to time, was
in some minds loosely associated with atheism. Gamow even suggested steady-state
theory was attached to the Communist Party line, although in fact Soviet
astronomers rejected both steady-state and big-bang cosmologies as "idealistic"
and unsound. Hoyle himself associated steady state theory with personal
freedom and anti-communism.
Astronomers
in the United States found the steady-state theory attractive, but they
took a pragmatic approach. The rival claims of big-bang and steady-state
theory must be settled by observational tests. One test involved the ages
of galaxies. In a steady state, with continuous creation of matter, there
would be a mixture of young and old galaxies throughout the universe.
In a big bang, with only an initial creation, galaxies would age with
time. And astronomers could look back in time by looking at more distant
galaxies, for observing a galaxy a billion light-years away meant seeing
it in light that had left it a billion years ago. Observations reported
in 1948 purported to find that more distant galaxies were indeed older.
Score one for the big bang. Bondi and Gold reviewed the data carefully,
and in 1954 they showed that the reported effect was spurious. Score one
for steady state. The age test might be able to distinguish between the
rival theories in principle, but in practice it could not.
A
nother
possible test
involved the rate of expansion of the
universe. In a big bang, the expansion rate would slow; in a steady state
universe it would remain constant. Data from the Mount Wilson Observatory
seemed to favor the big bang, but not certainly enough to constitute a
crucial test.
Meanwhile
there was a solution to the embarrassing calculation that put the age
of a big-bang universe less than the age of the solar system. Walter Baade
showed that estimates of the distances to galaxies had mixed together
two different types of stars (
as
explained here
). As a result, the size of the universe had been underestimated
by about a factor of two. If galaxies were twice as distant as previously
thought, then calculation with the observed rate of expansion gave an
age of the universe twice as great as previously calculated safely
greater than the age of the solar system.
That
argument against the big-bang universe thus dissolved
.
T
he
most serious challenge
to steady-state theory came
from the new science of radio astronomy. Fundamental knowledge in the
techniques of detecting faint radio astronomy signals advanced greatly
during World War II, especially with research on radar and especially
in England. After the war, research programs at Cambridge, at Manchester,
and at Sydney, Australia, built radio telescopes to detect signals from
outer space. They dominated radio astronomy for the next decade.
The
program at Cambridge was led by Martin Ryle, who in 1974 would receive
the Nobel Prize in physics for his overall contributions to radio astronomy.
In 1951 Ryle believed that radio sources were located within our galaxy,
and hence were of no cosmological interest. But over the next few years
he became convinced that most of the radio sources he was detecting were
extragalactic. His observations, then, could be used to test cosmological
models. Ryle argued that his survey of almost 2,000 radio sources, completed
in 1955, contradicted steady-state theory, because more distant/older
sources seemed to be distributed differently from nearby ones. But he
overstated the significance of his initial data. Only after more years
of work would radio observations argue strongly against steady-state theory.
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MORE
about Radio Astronomy
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